PROJECT SUMMARY. Trisomy 21 (T21) is the most commonly occurring chromosomal abnormality in the human population and gives rise the condition known as Down syndrome (DS). Individuals with DS experience a unique disease spectrum relative to the typical population, whereby they are protected from certain conditions such as solid cancers and predisposed to others including Alzheimer?s Disease, hematopoietic malignancies, and a host of autoimmune disorders. Despite decades of study, the molecular underpinnings of DS pathophysiology remain unclear. We recently reported that T21 results in constitutive activation of the interferon (IFN) response, likely due to the presence of four of the six IFN receptors on chr21. The central hypothesis of this project is that hyperactive IFN signaling contributes to the pathophysiology of DS. This proposal outlines the cause-effect investigations necessary to define the nature of the relationship between IFN hyperactivity and DS-associated phenotypes using mouse models of DS with triplication of the IFN receptors. Aim 1. To characterize the impact of immune stimulation on DS phenotypes. Using a mouse model of DS with triplication of the IFN receptors, termed Dp16, we will characterize the response to immune stimulation with the TLR agonist, poly(I:C), IFN?, or IFN?, on 1) the development, growth, and overall health of these animals, 2) expression of inflammatory markers and relative abundance of multiple immune cell types, 3) markers of neuroinflammation and neurodegeneration, and 4) learning, memory and behavior. Aim 2. To define the contribution of IFNR gene dosage to DS phenotypes. The four chromosome 21- encoded IFN receptors are located in a contiguous cluster in chromosome 16 in mice. To test the impact of IFN receptor copy number on immune sensitivity, we will cross Dp16 mice to a novel strain we have created in which all four IFN receptors have been deleted, termed IFNR4KO, to create mice with either two (?normalized? Dp16) or three (?trisomic? Dp16) copies of the IFNR cluster. We have also generated a strain with a segmental duplication of the IFNR cluster, dubbed IFNR3X, which we will use to test if triplication of this region is sufficient to drive DS phenotypes. Aim 3. To test the effect of anti-IFN therapies on immune hypersensitivity and DS phenotypes. We will pre-clinically assess the therapeutic potential of three independent ant-IFN strategies: 1) a novel JAK1-specific inhibitor currently being developed for treatment of autoimmune conditions, 2) an IFN-neutralizing Vaccinia protein being developed for HIV-associated neurological disorder (HAND), and 3) an ?IFN-kinoid? that triggers production of IFN?-neutralizing antibodies currently under development for treatment of systemic lupus erythematosus (SLE).